Usually, in the case of an electric vehicle, such as an electric car, a fuel-cell vehicle, or a hybrid electric car, a high-voltage power supply circuit is insulated from low voltage vehicular electric component circuits. For example, the high voltage power supply circuit may be a battery at a voltage of about 300V DC. The low-voltage vehicular circuits are typically in the range of 0 V DC˜14 V DC. In a conventional grounding detector for detecting grounding of the high-voltage power supply, a coupling capacitor is connected at one end of the coupling capacitor to the positive or negative side of the high-voltage power supply circuit, and a square or rectangular waveform pulse is applied to a measuring point at the other end of the coupling capacitor in order to facilitate detecting the occurrence of grounding of the high-voltage power supply. Typically, in the environment of an electric powered vehicle, high frequency and low frequency noise becomes super imposed on the rectangular waveform pulse.
Japanese Kokai Patent Application No. 2003-274504 discloses a grounding detector in which the noise is removed using a band pass filter equipped with a high-pass filter and a low-pass filter. A grounding detector disclosed in that patent application that is designed to remove high-frequency and low-frequency noise not attributable to the rectangular waveform pulse that is otherwise superimposed on the voltage generated at a measuring point. The noise is removed using a band pass filter equipped with a high-pass filter and a low-pass filter. An offset circuit is used to superimpose a direct current element on the voltage after the noise is removed. When the high-frequency and low-frequency noise superimposed on the voltage generated at the measuring point is removed using the band pass filter, the direct current element is also removed, and the voltage output from the band pass filter becomes a voltage signal that oscillates around 0 V. However, because a control circuit, such as a CPU, is usually incapable of detecting a voltage lower than 0 V, an offset circuit is used to superimpose a direct current element onto the voltage after the noise has been removed so as to offset the voltage output from the band pass filter and to convert it into an electric signal in a voltage range (for example, 0 V˜5 V) that the control circuit can read. Thus, the amplitude of the voltage that has been offset may be detected using an electronic device, such as a CPU. A grounding of the circuit connected to the power supply is detected by the CPU based on the amplitude value of the detected voltage.